Is there a serious risk that the Atlantic current that warms Europe will slow as the planet warms? Yes, according to the most detailed computer simulation to date – but the likelihood of this scenario is still very uncertain.
“We have demonstrated with our current setup that this is indeed possible,” says rené van westen at Utrecht University in the Netherlands.
Currently, warm water that is extra saline due to evaporation flows north from the tropics along the surface of the Atlantic Ocean, keeping Europe warmer than otherwise. As this water cools, it sinks because its high salinity increases its density. Then it flows back into the tropics and along the ocean floor into the Southern Hemisphere.
This is known as the Atlantic Meridional Overturning Circulation or AMOC. Studies of past climate show that episodes of dramatic cooling across Europe during the last 100,000 years have been associated with recessions or complete cessation of upwelling currents – a so-called tipping point, where small changes propel one system into a different one. States can change.
The reason for this is considered to be the melting of ice sheets. If a lot of fresh water enters the North Atlantic, it reduces the salinity and density of surface water, meaning less of it sinks.
But modeling it has proven difficult. Most simulations of shutdowns involve adding unrealistically large amounts of fresh water at once. And in some recent simulations with more advanced models, no shutdowns have occurred, leading some to doubt whether this is a potential tipping point.
Now Van Westen’s team has carried out the most sophisticated simulation to date, requiring a total of six months on the Netherlands’ national supercomputer, called Snelius. It was too expensive, he says.
Unlike previous simulations, the team added fresh water gradually rather than all at once. This created a positive feedback that amplified the effect: less water sank due to lower salinity, causing less salty water to flow north, reducing salinity even further.
This eventually shut down the overturned circulation, causing temperatures to rise in the Southern Hemisphere, but temperatures to fall in Europe. For example, in the model, London is cooler by an average of 10°C (18°F) and Bergen in Norway by 15°C (27°F). Other consequences include local sea level rise in places such as the US East Coast.
Furthermore, some of the changes seen in the models before the collapse match changes being observed in the real Atlantic in recent decades.
However, to produce this collapse, the researchers had to run the model for 2500 years. And they had to add large amounts of fresh water – less than in previous simulations, but still about 80 times more than is entering the ocean from melting Greenland’s ice sheet. “So this is absurd and not very realistic,” says Van Westen.
Furthermore, the simulations did not include any global warming. The team now plans to re-run the simulation to include this.
Says, “This is the most advanced model where such an experiment has been done.” Peter Ditlevsen at the University of Copenhagen, Denmark, who co-authored a 2023 study that predicted the Atlantic Overturning Current could collapse between 2025 and 2095, based on changes in sea surface temperatures.
While models suggest it would take a lot of fresh water and several centuries to stop the overturning circulation, there are reasons to think that climate models underestimate the risk of non-linear changes like the Atlantic tipping point, Ditlevsen says.
He says that to make the calculations feasible, climate models would have to divide the world into large cubes, the effects of which would be smooth. Models are also tuned based on how well they simulate the climate of the 20th century, when there was a linear relationship between greenhouse gas emissions and resulting changes, which may not hold true in the future.
“We should expect models to be less sensitive than the real world,” says Ditlevsen.
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